Tapered energy absorbing radome portion
FIELD OF THE INVENTION AND RELATED ART STATEMENT 
     The present invention relates to a radome. The radome is a dust-proof and water-proof cover for an antenna. 
     FIG. 7 is a partially broken perspective view of a conventional radome. 
     An antenna element 1 is fixedly mounted to a post or the like through a structural member 2. The structural member 2 is commonly formed of metal and serves as a support member of a radome member 3 in this example. The radome member 3 is positioned intermediate the antenna element and the outside space and the electric wave is transmitted through the radome member 3. Accordingly, the radome member 3 is manufactured in consideration of electrical characteristics (such as, for example, transmissivity and reflectivity of the electric wave) and mechanical strength (such as, for example, a wind pressure tight characteristic). 
     In the example of FIG. 7, in order to approximate the transmissivity of electric wave in a portion 3a through which the electric wave from a radiation plane 4 of the antenna element of the radome member 3 is transmitted to 100%, the portion 3a is adapted to be formed of a sufficiently thin film structure or is formed of a multilayer structure made of materials having different dielectric constant and having a large transmissivity in a particular frequency range. On the other hand, a portion 3b which is joined to the structural member 2 of the radome member 3 is formed of hard material to settle the strength. Generally since the hard material has the high electric constant, metal forming the structural member 2 is covered by the dielectric having the high dielectric constant. That is, a transmission path having a small loss is formed. Further, an end surface of the radome member 3 is in contact with the metal portion. 
     In the conventional radome, a current induced in the structural member 2 by an excitation current of the antenna element 1 flows as shown by the broken line 5 of FIG. 7. An electromagnetic wave generated by the induced current is propagated in a junction 3b of the radome member and reflected by the contact surface with the metal portion. That is, there is a problem that the junction 3b serves as a kind of antenna and disturbs the radiation characteristic of the antenna element 1. This phenomenon can not be avoided as far as the radome member is formed of the dielectrics and the structural member is formed of metal. 
     When this phenomenon occurs, the frequency dependency of the radiation pattern of the electric wave from the antenna element becomes remarkable, particularly in the case of an applied apparatus of an amplitude comparison monopulse system, the accuracy of the system is deteriorated. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a radome capable of suppressing the antenna effect in the junction of a radome member and a metal portion. 
     In order to solve the above problems, the radome including a radome member formed of a dielectrics to protect an antenna element from outside space and a structural member formed of metal for supporting the antenna element at the center and to which the radome member is joined at the periphery of the antenna element to support the radome member, is characterized by the provision of a portion of the radome member through which an electric wave is transmitted and which is formed of material through which an electromagnetic wave having a frequency identical with that of the electric wave is transmitted and a junction formed between the radome member and the structural member and having a structure for absorbing the electric wave. 
     In the prior art, the reduction rate α of the electromagnetic wave in the radome member by an induced current in the structural member is very small. Accordingly, the electromagnetic wave propagating the radome member is reflected by a contact surface of the radome member and is emitted to the outside space. Consequently, the radiation characteristic of the antenna is changed. 
     In the present invention, the junction of the radome member and the structural member includes the electric wave absorption structure containing electric wave absorption material and the electromagnetic wave propagating the portion is attenuated greatly. Consequently, the electromagnetic wave generated in the dielectrics of the radome member is controlled due to the induced current flowing through the structural member being radiated in the outside space. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus, are not limitative of the present invention, and wherein: 
     FIGS. 1 to 4 are partially broken perspective views of preferred embodiments of an antenna provided with a radome according to the present invention; 
     FIG. 5 shows an example of a directivity characteristic of an antenna having no radome; 
     FIG. 6 shows an example of a directivity of an antenna having a radome; and 
     FIG. 7 is a partially broken perspective view of an antenna having a conventional radome. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 is a partially broken perspective view of a preferred embodiment of an antenna having a radome according to the present invention. 
     An antenna element 1 which is a monopulse antenna is fixedly mounted to a support or the like by a structural member 2. A portion 3a of a radome member 3 through which an electric wave from a radiation plane 4 of the antenna is transmitted is formed of a dielectrics having a transmissivity for an electric wave of about 100%. On the other hand, a junction 3b formed between the radome member 3 and the structural member is formed of two portions A and B. The first portion A is formed of a dielectrics having a strength capable of supporting the radome member 3 sufficiently and the radome member 3 is joined to an outer surface of a cylindrical portion 2a of the structural member 2. An end surface 3c of the radome member 3 is not in direct contact with an annular shoulder 6 of the antenna support 2 and a second portion B of the junction 3b constituting an annular electric wave absorption member is disposed between the end surface 3c and the annular shoulder 6. An electromagnetic wave generated by an induced current 5 flowing through the structural member 2 is attenuated in the second portion B constituting the electric wave absorption member. Material containing carbon, carbonyl iron powder or the like can be used as the electric wave absorption member. 
     FIG. 5 is a graph showing an example of a directivity characteristic of an antenna formed of only a monopulse antenna element and a structural member and having no radome. 
     FIG. 6 is a graph showing a directivity characteristic of an antenna formed of a monopulse antenna element, a structural member and a radome. Solid line shows the case having the conventional radome of FIG. 7 and broken line shows the case having the radome according to FIG. 1 embodiment. It is apparent from the figure that the directivity of the conventional radome is disturbed while the directivity of the antenna having the radome according to the present invention approximates the directivity without radome. 
     FIG. 2 is a partially broken perspective view of another preferred embodiment according to the present invention. 
     This embodiment is identical with the embodiment of FIG. 1 except only the structure of the junction between the radome member 3 and the structural member 2. The junction C between the radome member 3 and the structural member is formed of a dielectrics in which powder of the electric wave absorption member is mixed. Carbon, carbonyl iron powder or the like can be used as the electric wave absorption member. Mixing the electric wave absorption member into the dielectric (plastic) can be easily made using the common technique. 
     It is preferable that a boundary 3aC between the portion 3a through which the electric wave of the radome passes and the junction C is tapered as shown in FIG. 2. In this case, the function of the junction C as a termination element for the electromagnetic wave in the dielectric is great and the radiation of the electromagnetic wave from the radome member to the outside space is reduced. 
     FIG. 3 is a partially broken perspective view of a still further embodiment of the present invention. 
     In this embodiment, only a shape of a boundary between the junction D of the radome member 3 and the structural member 2 is different from the embodiment of FIG. 2. Specifically, the junction D of the radome member 3 is embedded into a deep annular groove formed in the structural member 2. The junction D formed of the dielectric is reinforced by metal from the outside and accordingly the mechanical strength is large. 
     FIG. 4 is a partially broken perspective view of a still further embodiment of the present invention. 
     In this embodiment, only a shape of a boundary between the junction E of the radome member 3 and the structural member 2 is different from the embodiment of FIG. 2. An annular flange is provided in the radome junction E and is fixed to the structural member 2 by an annular mounting member 7 so that the radome member 3 is fixed to the structural member 2. 
     The effects of the present invention are as follows: 
     The junction includes the electric wave absorption structure and accordingly the attenuation factor of the electromagnetic wave in the transmission path is large. Thus, the junction is operated as the terminator. Consequently, an amplitude of the standing wave existing in the junction which is an element disturbing the antenna pattern is made small and the disturbance of the antenna pattern is also made small. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.